An electric energy generator is indispensable in the modern industrialized society, and particularly in recent years, attention has been paid to a lithium battery or lithium ion battery which employs a compound containing lithium element on its negative electrode or positive electrode as a high performance battery from the viewpoints of its high voltage and high energy density.
A basic function of an electrolyte which is one of important battery components is to transport ions in an electric energy generating reaction (electrochemical reaction). Examples of characteristics required for the electrolyte used on high performance batteries are excellent ion transporting capability, high resistance to oxidation, flame retardance, improvement in charge and discharge cycle of secondary batteries and prevention of dendrite of lithium metal in charging.
As the electrolytes which are known to be suitable for high performance batteries, there are organic compounds such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and .gamma.-butyrolactone.
However electrolytes which have been used hitherto do not satisfy the characteristics mentioned above. Though a battery having high electromotive force which employs a N-fluoropyridinium salt as an active material for positive electrode has been proposed (JP-A-7-6756), when a conventional electrolyte is used, there are problems that battery deteriorations such as lowering of discharge voltage and lowering of discharge capacity occur under high temperature condition.
Accordingly, since high polarity generates excellent ion transporting capability and the above-mentioned known compound for electrolytes contains, in its molecule, only one carbonyl group which is a polar functional group having electron attractive property and represented by: ##STR1##
the present inventors have expected that the above-mentioned problems can be solved by using, as an electrolyte, a dicarbonyl compound having two carbonyl groups in its molecule, and have challenged to use a dicarbonyl compound having the basic structural formula (II): ##STR2##
However as shown in the following reaction scheme (1): ##STR3##
since the dicarbonyl compound represented by the basic structural formula (II) exists both in keto form (II) and enol form (II') at equilibrium, when used as an electrolyte, there is a fatal defect such that the enol form of the dicarbonyl compound reacts with a metal such as lithium of a negative electrode to generate hydrogen gas.
As a result of intensive study, the present inventors have found that when hydrogen atoms of the methylene group in the above-mentioned dicarbonyl compound are substituted with halogen atoms having strong electron attractive property, the resulting dicarbonyl compound does not become enol form, and thus the above-mentioned problem of hydrogen generation by reacting with a metal such as lithium can be solved. Further the present inventors have found that by introducing halogen atoms, the dicarbonyl compound can be stabilized and endowed with flame retardance and high resistance to oxidation.
Namely an object of the present invention is to obtain an electrolyte which has high polarity, high resistance to oxidation and flame retardance and is suitable for an electric energy generator to improve charge and discharge cycle and inhibit dendrite of metal lithium. Another object of the present invention is to obtain an electrolyte suitable for a battery in which N-fluoropyridinium salt is used as an active material for positive electrode having a high electromotive force.